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BQ24070EVM: Charging deeply discharged battery

Yusi Ou42
Prodigy 30 points
Part Number: BQ24070EVM
Other Parts Discussed in Thread: BQ24070

Hello,

I am testing the BQ24070EVM and am having trouble charging a deeply discharged battery. The battery is a 3.7V LiPoly which has a safety circuit that drops the output to 0V when the battery is below 3V. There is another thread on the forum about this: which never received an answer.

 

I followed the test procedure in the EVM manual and made the following observations:

1. With the battery plugged in and DC power supply off, VOUT is at 0.20V.

2. After supplying 5.25V to IN, VOUT at 0.20 V but all three LEDs were on, indicating that the system was in the pre-charge state. 

3. After 5 minutes, VOUT still at 0.20 V and precharge LEDs still on.

4. I unplugged the discharged battery and power supply, and connected a charged battery. VOUT at 4.4V. Turned on power supply, STAT1 and PG LEDs on, VOUT at 4.4V.

5. I unplugged the charged battery and power supply, and connected the first battery again (its output now at 3V). Turned on power supply, STAT1 and PG LEDs on, VOUT at 4.4V.

6. Discharged the first battery fully again (output 0V), reconnected to EVM. VOUT at 0.20V. Removed the load, then replaced it (as suggested by the test procedure). Turned on power supply, STAT1 and PG LEDs on, VOUT at 4.4V.

 

Is there a way to "unlock" the battery for charging without disconnecting it or the load? Would adding a capacitor on DPPM to delay the short circuit help? The power management system will be enclosed inside a device so I would not be able to physically disconnect the battery/load when the battery becomes discharged. 

Thanks!

  • 0
    TI__Mastermind 21590 points

    Hi,

    Due to the fact that the battery is completely discharged, this is mostly due to the battery preconditioning safety timer timing out as explained below. Please consider extending this time by increasing R(TMR).

    Regards,

    Raheem

  • 0 in reply to Raheem
    Prodigy 30 points

    Hi,

    I tested again with the default pre-charge timer setting (1800 s), and don't believe this is the issue but would like some clarification.

    I connected the discharged battery (0V), and the system goes into short-circuit mode (VBAT = 0V, VOUT = 0.25V, VIN current = 15 mA), but all STAT LEDs were on, indicating pre-charge. After 30 minutes, I did not see any change indicating fault (LEDs still on). I disconnected/reconnected the load and the system immediately began fast-charge (VBAT = 2.8V, VOUT = 4.4V, VIN current = 1.3 A, STAT1 and STAT3 on). I repeated the test with another discharged battery and waited 15 minutes before disconnecting/reconnecting the load and the system went directly into fast-charge. I repeated the test again, but immediately disconnected/reconnected the load after connecting the discharged battery, and the system went into what I think is pre-charge (VBAT = 2.8V, VOUT = 0.25V, VIN current = 250 mA, all STAT LEDs on), and then after about 15s, fast-charge.

    I think that the IC is successfully reviving the battery in the first two cases, but I didn't think this was possible in short circuit. Do you have any insight? The EVM has a capacitor on DPPM, and the system does not go into short circuit mode when I hot-plug a charged battery (VBAT > 3V).

    What I understand from the operation flow chart on pg. 16 of the BQ24070 data sheet is the IC should be able to revive the battery and continue onto fast-charge without having to disconnect the load. Is this correct? If so, then is there any other reason why the IC might be stuck in short circuit but still able to pre-charge? Is the IC just not compatible with a battery that has this sort of safety circuit?


    Thanks!